How to fill a grease pencil ring?
UPDATE: As of version 2.91 we now have a Holdout option in the material settings which makes life much easier:

Create a new material, check Fill and Holdout
Use the paint bucket to fill in any area you want to be see-through, or just draw the inside of the ring using this material.


Old answer:

Create a new layer and name it &quot;hole&quot;

Draw a filled circle matching the ring's inner circle

Turn down the opacity of the &quot;hole&quot; layer to 0.000

Switch to the fill layer (that filled the entire ring) and enable mask

Add &quot;hole&quot; to the mask list and invert it by clicking on the square/circle icon